text:
Why would the Pakistan airspace closure cancel flights not headed to Pakistan itself?
The India-Pakistan feud has heated up again, with Pakistan closing its airspace as a result.
I know many international routes fly over Pakistan, and that the closure would cause issues, but this report surprised me:

Thousands of people were also stranded by affected airlines that not only land in Pakistan, but fly over its airspace -- one of the major routes from Southeast Asia into Europe.
Thai Airways announced that all its European routes "departing near midnight of 27 FEB through early 28 FEB" were canceled "due to sudden closure of Pakistani airspace as a result of tension between India and Pakistan."

Why would the airspace closure force flights over it to cancel entirely? I would think you would just reroute them around it.

Travelling through the middle east is not easy, not even above the skies. There are several closed airspaces and a few you want to avoid due to terrorism and an increasing capacity to hit high altitude targets.

Consider the fact that all flight routes that were planned had to be handed in again and be accepted. Changes result in necessary fuel and new time slots for takoff and landing airport. This has to be recalulated and might take some time

To give an example of how flights can be affected by this in ways to make them impossible, Iranian airspace is closed from sunset to sunrise (unless things have changed recently).
Any aircraft that due to the closure of Pakistani airspace would need to cross Iranian airspace and be unable to do so because of that closure now needs to be cancelled or rescheduled.
Also, if the diversion around Pakistani airspace would stretch the endurance of an aircraft beyond what fuel it can carry, it cannot fly the route.
Or the diversion would cause such a delay in the schedule that it causes too much problems with the overall scheduling of the airline. Such things tend to have a ripple effect. One flight gets delayed by several hours, dozens of others get delayed as a direct result, hundreds more can suffer delays in the end.
Also, some airports only operate during daylight hours. A diversion around Pakistan might mean the flight can't make it to one such airports before sunset and thus can't land at its intended destination. Now the airline has 2 options, divert to another airport, arrange for bus or train transport for all the passengers, and in the morning yet another hop to the intended destination to pick up the new passengers, or cancel the flight.

Or the airport doesn't operate late nights, such as SYD.

Iranian airspace is closed from sunset to sunrise * I might be wrong but I see many landing scheduled at night there

I would love to know more about Iranian airspace, maybe I'll post a different question. Flight paths over Iran, Iraq, and Afghanistan are typically restricted to one of a few routes.

I'd add that even if they can fly around Pakistan airlines are particularly cautious after MH17 was shot down over Ukraine. It's easier to just cancel the flight then guess if your leaving enough of a buffer zone. Especially when tensions are escalating between 2 countries.

@JohnRay: MH17 didn't involve any buffer zones - it was shot down while flying directly over an active warzone.

@Sean - Sorry, I meant that since MH17 airlines now want to leave a large buffer zone around any conflict areas.

Iranian airspace is closed from sunset to sunrise this doesn't seem true. For example, QR195 departs DOH at 01:40 and heads for Brussels flying over Iran.

unless things have changed - that at least used to be true? What's that about?

Not mentioned in the other answers is simply logistics coordination.  If you can't fly over Pakistan, that suggests that maybe you have to fly somewhere else.  Perhaps flying around means an overflight of China or Kyrgyzstan.  Do they charge overflight fees?  Do they require prior permits?  Even if the money involved isn't huge, starting up a new route may mean the first time your airline had to work with that country.  I'd expect that could take a few days.
Even if you already have a relationship with the country from other routes, adding permits and having the accountants approve the route choice would still be required.
https://www.jetex.com/overflight-fees-in-asia-pacific/

Interestingly, right now Thai Airways is largely flying over China, Kazakhstan, Russia, and Belarus, while EVA Air is using a route over India, the Arabian Sea, Iran, and Turkey.  This speaks to the fact that logistical factors will differ between airlines, and different decisions will be made.

It also appears that Thai Airways avoided Iran even before the Pakistan closure.  This may be because Iranian overflight fees are relatively expensive.

There are multiple reasons for cancelling instead of rerouting:

Take-off & landing slot assignments

You may have a pair of slots available and your regularly scheduled flight uses them. Rerouting the flight will take longer and you may no longer make your destination landing slot

Departure & arrival gate assignments

Likewise, you may no longer have a gate assignment available at the destination airport because the reroute takes too long.

Aircraft range.

If your flight is near the maximum range by overflying Pakistan, rerouting around it could push the plane past its maximum allowable range.

If range is the problem, why not just make a technical stop somewhere like India or Iran?

@Sean it's not so simple. You need to arrange the landing and take-off slots, you also increase costs and schedule significantly.

There was an extensive report on the grounding of Thai Airways flights in The Guardian today. To quote:

Thai Airways said later on Thursday it would resume flights after gaining permission from China to use its airspace for nearly a dozen flights to Europe set to leave on Thursday afternoon and Friday morning.

Quite simply, Thai lacked the necessary permissions.
The Chicago Convention on International Civil Aviation established a general right for overflight by foreign aircraft, but many states nonetheless require airlines to acquire permission in advance – including China.

Why not fly over India and the Arabian Sea instead?

That would also explain why certain airlines, most notably the Taiwanese airlines China Airlines and Eva Air, don't fly over PRC airspace. Given the political sensitivity between the two countries at the moment and the pressure the PRC is imposing on airlines regarding the wording of Taiwan, it's highly doubtful that any Taiwan-registered airline would ever be granted permission to use PRC airspace.

@Sean:  I noted in my comment on another answer that Iranian overflight fees are relatively expensive, and Thai Airways avoided Iranian airspace even before the closure.  If you don't want to overfly Iran, and you don't want to overfly the Middle East, then going towards the Arabian Sea doesn't help you that much.

@MichaelDodd Taiwan-based airlines operate regularly scheduled flights between Taiwan and PRC airports. I wouldn't be so sure overflight would be restricted.

@cpast I may be mistaken, or at least the situation may have changed since I last flew to Taipei. When we visit my wife's family our usual choice of flight is China Airlines CI70, with CI69 as the return flight. While CI69 still flies past Japan and over Russia, arguably taking a longer path to avoid Mainland China, CI70 does seem to skirt the Southern PRC on its way to the Hong Kong ATC waypoint.

@cpast I was initially under the impression that flying over a country's airspace without landing and flying to a destination within that country were two different set of circumstances, where one may be permissable and the other denied.


text:
Are helicopters impacted by fog more than airplanes?
With the crash of Kobe Bryant's helicopter, it seems a lot of focus has been placed on the helicopter flying low in fog (under "special VFR"). However, I've seen several suggestions that the helicopter should not have been flying at all

Weather reports indicated that a low cloud cover was present on the day of the crash, further complicating flying. Bryant's pilot was operating the craft under a special clearance for poor weather conditions and reportedly had experience in such situations.

The pilot was IFR rated, but if the investigation proves pilot error it raises the question if helicopters should be more limited in fog. Wired makes it sound even more dire

Flying without sight of the ground or other landmarks can quickly become dangerous, Whitcomb says, because helicopters are not inherently stable. Just staying level and on course demands working four controls at once, using your hands and feet, and keeping track of how each input affects all the others. If you lose sight of where you are and you don’t use your instruments properly, you may not realize you’re turning, or dropping, or even upside down. “You’ll get disoriented within seconds,” Whitcomb says.
Navigating by instrument can also be mentally taxing, says Scott Shappell, who chairs the Department of Human Factors and Behavioral Neurobiology at Embry-Riddle Aeronautical University. He compares visual flight conditions to driving down a familiar highway. Flying by dashboard is more like driving through Rome for the first time—while working a stick shift. “It’s definitely more work,” Shappell says. “The risk goes up.”

Are helicopters really that much more impacted than airplanes by fog?

I would have put another introduction as it may bring speculation about an ongoing investigation.

No.  If anything, given a helicopter can fly a much slower cruise speed than an airplane typically does, they do not require the visibility minimums which fixed wing aircraft do.  VFR weather minimums for a helicopter in uncontrolled airspace are 1/2 mile versus 1 mile for airplanes.  Be that as it may, a helicopter is still vulnerable to Loss of Control Inflight (LOCI) due to pilot error just like any other aircraft.

Flying speed has nothing to do with how quickly you find yourself disoriented in whiteout conditions.

No but it does allow a helo to fly slower and increases available time to react to hazards, thereby allowing for more restrictive visibility minimums under Visual Flight Rules, which I think my post clearly spelled out.

I will up-vote anyways for the first paragraph. Speculation is a hard pill though.

I'm under the impression that speculating on accidents that are still under investigation is totally out of bounds here. Correct me if I'm wrong.

Depends on how or what you are speculating on.

Please avoid speculating on on-going investigations! Note I've removed the last paragraph

Yes, wired is spot on. Helicopters are more unstable than fixed wing aeroplanes, especially at low speeds. The instability has a relatively large period, so helicopter pilots can attain the feedback skills in order to actively maintain attitude and position - if the visual feedback cues are from peripheral vision, which is our quick motion detection mechanism. Central vision is more accurate but slower to detect changes, and instrument flying without the help of peripheral vision is challenging.
Having said that, the S76 has a Stability Augmentation System which helps in retaining helicopter attitude, by providing swashplate inputs if attitude changes while not being commanded by the pilot. Whether an instrument rated helicopter pilot with SAS engaged should be more limited than a fixed wing pilot, remains an open question.

how is a Helicopter "More unstable" than a fixed wing? Especially at low speeds.

only one can stop and hover if confused, I think.

@Skyhawg In low visibility, you may not realize you aren't flying straight and level.  This is especially true while hovering, as it is easy to start drifting in some direction, doubly so if you have no visual cues.

@skyhawg The tail and wings of most fixed wing aircraft provide a natural stabilizing force. Even without autopilot you can let go of the stick and it will be ok for quite a while in good conditions. A helicopter is nothing like that, it needs near constant corrections. This tendency makes IFR in helicopters much more dangerous IMO.

@Skyhawg A helicopter can hover and a fixed wing cannot. But the hover is an unstable event: the pilot needs to provide continuous stick inputs in order to keep the helicopter level, it wants to fall away in pitch and roll all the time.

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text:
Why didn’t the space shuttle have flight control manual-reversion capability?
The Wikipedia article on STS-1 (the first orbital space shuttle mission), in a discussion of all the problems and malfunctions occurring during the mission, mentions that, due to an unexpected shockwave during booster ignition, Columbia’s rear bodyflap (part of its pitch control system) was (unbeknownst at the time to either Mission Control or the crew) bent upwards beyond the point where damage to the shuttle’s hydraulic systems would have been expected.
According to the article, had Columbia’s hydraulic lines actually been breached, resulting in a total loss of hydraulic fluid, this would have rendered the shuttle uncontrollable as soon as the aerodynamic forces on the orbiter during reentry exceeded the control authority of its reaction-control system.
Small- to medium-sized aircraft with hydraulically-actuated flight controls essentially always have some form of manual reversion capability in case a no-hydraulics situation occurs, where the primary flight control surfaces (always the elevator and ailerons, and sometimes the rudder as well) are operated manually by the pilots, usually by control cables attached to the pilots’ yokes and pulling (depending on the aircraft) either on the control surfaces directly or on servo tabs which then generate aerodynamic forces which move the control surfaces.  (Large aircraft generally don’t, even though even partial control authority would still be better than nothing, but that’s another question.)
To the best of my knowledge, the largest Western aircraft to have flight controls with manual-reversion capability is the Boeing 737 MAX,1 which has provisions for full manual control of the ailerons and elevator, plus a small amount of undocumented manual rudder control (as the rudder is rarely or never used during normal flight, manual rudder control is far less critical to have than manual elevator and aileron control, and using a manual aileron-and-elevator system rather than a manual three-axis system allows for a simpler system; however, due to the design of the 737’s rudder system, the rudder can nevertheless be manually moved slightly by a sufficiently large amount of force on the rudder pedals).  The 737 MAX is considerably larger than a shuttle orbiter, with a maximum weight of 88,300 kg, as compared to the shuttle’s empty weight (as its flight control capability, or lack thereof, would only be relevant during descent, when it would generally be empty, and, therefore, at its lightest) of 68,600 kg; even with the shuttle orbiter at its maximum allowable weight (to allow for the possibility of a hydraulic failure during an abort scenario while carrying the absolute heaviest possible payload) of 109,000 kg, it would be less than a quarter again the size of a 737 MAX 9, which would seem to indicate that including manual-reversion capability for the shuttle’s flight controls would have been doable.
Why, therefore, wasn’t it included?

1 The Ilyushin Il-62 is almost twice as heavy as the 737 MAX 8, with an MTOW of 165,000 kg, and has a fully mechanical flight control system (no hydraulics even in normal operation); however, it was a Soviet aircraft, making it somewhat unlikely that the space shuttle design teams could have gotten access to its technical data (especially given the heavy involvement of the U.S. Air Force in the design and development of the shuttle - for instance, the shuttle would have had much smaller wings were it not for the design requirement to launch into a polar orbit to either deploy a U.S. spy satellite or kidnap a Soviet one, and then reenter and land after just one orbit, with the consequent large crossrange requirements).

I suggest this gets moved to SpaceExploration.SE, there are people with vast knowledge on the Space Shuttle program there and you're much more likely to get a definitive answer.

@GdD: I realise that, but this question is about the space shuttle's flight control system, which falls under the remit of Aviation.SE rather than SpaceExploration.SE.

I see where you're going, but I definitely think you'll get more definitive answers there @Sean. The case you've brought up has been discussed more than once.

The biggest reason (as is typically the case with space craft) was likely weight. It costs quite a bit of money to get even small amounts of mass into space, when talking about a vehicle keeping it light while maintaining a basic level of safety is key.
Unlike the 737 the shuttle lacked the yoke style controls that would be required for this kind of a system. The shuttle stick not only operated the flight control surfaces while on approach but also drove the reaction control system while in space. So there would be no way to operate a manual backup system even if there was one (without changing all the control surfaces).
The orbiter also had a triple redundant hydraulic system which was considered fail safe enough, and due to the highly automated flying of the orbiter hydraulic systems proved to be what was needed to make the system a reality. Cable controls for manual reversion in a flight regime barely flyable by humans would have largely been dead weight.

...why wouldn't one be able to use a joystick with a manual-reversion system?

@Sean you could if it was like the ones on say a P-51 but the shuttle system was different and had to control both the controls surfaces and the reaction rockets, the later of which cant really be driven mechanically.

Yes, but the question's about manual reversion as a backup in the event of a total hydraulic failure; unless the RCS thrusters were hydraulically-actuated, their being controlled by the same joystick as the flight control surfaces isn't really relevant.

@Sean while not directly related it was a design constraint to save weight and simplify operations in the orbiter. Sure you could have a cable based system but as noted above with triple redundant hydraulics any more would have been dead weight. If you listen to the above linked podcast they also cover the fact that the shuttles redundancies were largely to keep up appearances for the the public and most of the astronauts knew that a critical failure of any main system was not going to end well.

"barely flyable by humans" +1. Outside of testing, did a human ever fly an STS?

@Dave the redundancies were not for PR. On STS-79 an APU failed causing the loss of a hydraulic system. Saved by the redundancies. See page 23 https://ntrs.nasa.gov/api/citations/19970008143/downloads/19970008143.pdf  Your answer is correct, triple redundancy was good enough, adding a cable system would have been over the top.

@Mazura every mission, albeit briefly.

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text:
Are there any airplane designs that are (relatively) insensitive to the effects of airframe icing?
Snow and ice buildup on airplanes (especially the wings) is a well-known and feared hazard of cold-weather operations; it roughens the surface of the wings and changes their profile, which combine to enhance the tendency of the airflow over the wings to separate from the surfaces of same, and thereby decrease (often by a large amount) the amount of lift generated, change (sometimes markedly) the distribution of the remaining lift, decrease (frequently by several degrees) the attack angle at which the aircraft stalls, and alter (sometimes greatly) the aerodynamic forces acting on the aircraft’s flight control surfaces.  Snow and ice can also block (partially or completely) aircraft sensors, including those for such critical parameters as airspeed or engine thrust.
While icing is dangerous whenever it occurs, most icing-related accidents seem to fall into two categories:

Airplanes crashing on takeoff due to snow, ice, frost, etc., accumulating on the wings prior to takeoff as a result of no/inadequate/inadequately-recent anti-icing; in these accidents, the main factors are the loss of lift (causing the aircraft to take much longer and require much higher speeds to take off successfully, or even to be completely unable to lift off, period, or to climb out of ground effect after lifting off), the altered lift distribution (which, in most airliners, generally takes the form of a forward shift in the center of lift – as most transport aircraft other than the very smallest and slowest have rearward-swept wings, and the wingtips tent to freeze before the wing roots – resulting in a strong pitch-up tendency during rotation, or even in the aircraft spontaneously rotating early all on its own, and making it very easy to rotate the aircraft too far, all of which increase both drag and the risk of accidentally stalling the aircraft), and the decreased stall angle (which greatly increases the risk of a stall, especially when combined with the aforementioned pitch-up tendency, and especially especially because most stall warning systems sense only the aircraft’s attack angle, rather than directly detecting the airflow separation associated with an impending or actual stall, and are calibrated for ice-free conditions, meaning that, in icing conditions, the aircraft can stall at an attack angle well below that at which the stickshaker activates and the man in the box yells “STALL” at you).  Examples include Ozark 982, Continental 1713, Air Ontario 1363, USAir 405, and Air Florida 90 (the last of these was additionally affected by blocked engine sensors, which tricked the crew into choosing a power setting that would have been dangerously low even without airframe icing).
Airplanes falling out of the sky due to ice accumulating on their wings during flight as a result of encountering freezing rain or drizzle combined with non-use/inadequate use/inadequacy of the aircraft’s onboard deicing systems; in these accidents, the main factors are the changes in the aerodynamic forces imposed on the aircraft’s control surfaces (causing – as ice accumulation is frequently asymmetrical, and, even if, by some miracle, the aircraft does accumulate ice symmetrically, this symmetry is soon lost when ice starts to shed from the wings, as this process is random and essentially never symmetrical – a steadily-increasing force asymmetry on the aircraft’s ailerons; if the crew are flying the aircraft manually, they need to apply more and more force to their yokes just to keep the aircraft level, while, if the autopilot is flying the plane, the forces imposed on the aircraft will eventually exceed its control authority, causing its disengagement and an immediate violent roll, likely resulting in at least a temporary loss of control), the altered lift distribution, and the decreased total lift and stall angle (all of which make it more difficult to recover from the steep dives that generally follow a loss of control, and increase the likelihood that something will have some limit or another exceeded in the process).  Examples include American Eagle 4184 and Comair 3272.

Given the dangers posed by icing, are there any airplane designs that are relatively unaffected by snow/ice/frost/miscellaneous solid water accumulation?
I can think of a few possibilities:

Airplanes with very low wing loadings (i.e., very large wings for an aircraft of their weight) could be more likely to have sufficient excess lift to overcome the decrease in lift resulting from ice accumulation.
Delta-winged airplanes in subsonic flight do not rely on attached airflow to generate lift, but, rather, exploit flow separation and the resulting vorticies to generate a low-pressure area above the wing and produce lift; thus, they could reasonably be expected to be resistant or immune to the decrease in lift, altered lift distribution, altered flight-control forces, and increased stallability caused by flow separation consequent to icing, or even possibly see an increase in lift thanks to icing-enhanced flow separation.

Does "aircraft" include lighter-than-air and rockets?

@rackandboneman: Not in this case.  Edited question to make that clear.

The part about stall warning being calibrated to ice-free condition is incorrect. 14 CFR Part 25.207(e) requires stall warning in icing to be adequate for preventing stall. Of course, this is contingent on pilots actually turning on anti-ice/de-ice in icing conditions.


text:
What is this string on the 737?
The other day I was spotting at Miami International (MIA/KMIA) and noticed something hanging out of the fuselage.
A man with a green jacket is holding a device attached to a cable. Can somebody determine what that is?

That's the leash,  some cities require all airplanes to be leashed any time they are on the ground and require the operators to pick up after the planes as well.

@dotancohen If a 737 did the sort of things that need picking up on my lawn, I too would hope that someone actually picks up after it.

The ground crew has connected a headset to the communication jack outsiede for flight pushback communication with the flight crew. from b737.org.uk:

The External Power Hatch is located beneath the F/O's DV window. It is used by groundcrew to connect the Ground Power Unit and headset for pushback communications with Flight Interphone.
The service interphone is used by engineers to communicate with the service interphone stations inside the aircraft.

Here is a closeup of the connection point

Image from b737.org.uk
About flight interphone System

The flight interphone system is an independent communication network. Its
primary purpose is to provide private communication between flight deck
crewmembers without intrusion from the service interphone system. The ground
crew may also use the flight interphone through a jack at the external power
receptacle.

And here is a nose section of 737-200, showing it from a not so close angle.

Image from rigger.smugmug.com

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text:
Can radar detect a small flying device with MEMS microengines?
I have seen some MEMS micro engines and some micro turbines and I would guess they can be attached to some payload to fly. Is it possible for commercial/military radars to detect those micro flying devices?

Obviously the MEMS engine itself is not going to be detected: they're only a few millimetres across. The question entirely depends on what you attach it to.
Conventional radars have trouble detecting UAVs less than a couple of metres long, and replacing their power plants with micro turbines won't change that.
My neighbours at Aveillant already have a holographic radar system that can detect smaller targets such as UAVs more accurately. Again, changing the power plant technology won't change that.
Micro-turbines are not intended to replace propulsion systems, but to replace batteries, allowing them to use denser hydrogen, methane, or methanol fuel. As far as I've been able to find, they don't produce break-even power yet, but it's envisioned that they will become commercially relevant once they can produce more than 10 W (about .01 hp), in a few years' time. For comparison, a current, small, military UAV might generate tens of horsepower, and some experimental super-lightweight UAVs need as little as 5 hp, but with no payload or cargo.
TL;DR: (a) MEMS turbines aren't going to be relevant for UAVs for some years, and they'll have to be much bigger than they are now; (b) replacing the power plant won't make much difference to the radar signature.

Radar detection depends on the scattering of radar waves on objects. If we use the common X-band (8-12 GHz frequency) as an example, the wavelength $\lambda$ of the radar waves is in the order of 3 cm. Now the size of the target comes into play, expressed as a ratio α, defined as $$\alpha = \frac{\pi\cdot D}{\lambda}$$
with $D$ the diameter of the object to be detected.
α ≪ 1: Rayleigh scattering (small particle compared to wavelength of light). Radiation energy is diffused without a preferred direction, strongly dependent on wavelength.
α ≈ 1: Mie scattering (particle about the same size as wavelength of light, valid only for spheres)
α ≫ 1: geometric scattering (particle much larger than wavelength of light). Radiation energy is reflected with the angle of reflection equal to the angle of incidence.
Only for large α you will get a return signal which can be used for detection. Then we speak no longer of scattering, but of reflection. If your MEMS engine is just a few mm across and attached to a plastic airframe (which should be transparent to radar waves), it should be impossible to detect with conventional radars. Since the atmosphere is rather opaque for microwave radiation above 20 GHz, infrared or optical detectors will be needed to detect such an aircraft.
Also, the software driving the radar display is an important factor. Common aviation radars will filter out small, slow-moving objects, so even a metallic or carbon-composite airframe several centimeters across will not be detected.

Could you please explain the difference between scattering and reflexion, in this context?

It does not matter what type of engine is used; if the Radar is powerful enough and the flying object is near enough (making allowances for any low observable design) i.e. based on the Radar Cross Section, it will be detected by the Radar.
In case of small flying objects, the main problem is not in detection (Radars usually pick up bugs and such) but in recognizing the object in background clutter.
In this case, the military Radars have an edge as they are more powerful and are optimized for detection of small targets in ground clutter.

Some have even received anti-UAV updates (software mostly) in the past few years.

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text:
Is there any difference in landing minima between ICAO and FAA?
The FAA IFR Landing Minima Chart has both ceiling and visibility minima regardless of approach type (straight-in or circling). I believe it means that a pilot should meet the both ceiling and visibility minima although he conducts straight-in approach procedure.
But in my country (not USA, we follow ICAO regulations), landing minima for straight-in approach procedure consists of only 'visibility'.
So I guess there is difference on landing minima between ICAO and FAA. But I couldn't find that information.

Could you provide an example of an airport in your country with such an approach?

I live in Korea and I refer Incheon international airport(RKSI) approach procedure. But i don't know how to attach it. Sorry

In most cases, FAA approach charts do not provide a requirement for a ceiling as you say, but rather provide the minimum altitude that a pilot may descend to without having the runway environment in sight (the DH/MDA) and the minimum required visibility.1
In fact, as described in the FAA Instrument Procedures Handbook, Page 4-7 and in the AIM, section 5-4-20(e), air carrier pilots, who are restricted from even beginning an approach if the reported weather is below minimums, are only restricted by the visibility and may fly the approach  when the reported ceiling is lower than the minimum altitude on the approach plate (note that this does not apply to Part 91 operations).
The main reason for this is that a ceiling does not necessarily cover 100% of the sky over the airport environment, and it is quite possible to see the runway even when there is a reported ceiling.

References
FAA Instrument Procedures Handbook, Page 4-7 says:

14 CFR Part 135, section 135.225 states that pilots may not begin an
instrument approach unless the latest weather report indicates that
the weather conditions are at or above the authorized IFR landing
minimums for that procedure. 14 CFR Part 135, section 135.225 provides
relief from this rule if the aircraft has already passed the final
approach fix (FAF) when the weather report is received. It should be
noted that the controlling factor for determining whether or not the
aircraft can proceed is reported visibility. RVR, if available, is the
controlling visibility report for determining that the requirements of
this section are met. The runway visibility value (RVV), reported in
statute miles (SM), takes precedent over prevailing visibility. There
is no required timeframe for receiving current weather prior to
initiating the approach.

AIM, section 5-4-20(e) says:

e. Published Approach Minimums. Approach minimums are published for
different aircraft categories and consist of a minimum altitude (DA,
DH, MDA) and required visibility. These minimums are determined by
applying the appropriate TERPS criteria. When a fix is incorporated in
a nonprecision final segment, two sets of minimums may be published:
one for the pilot that is able to identify the fix, and a second for
the pilot that cannot. Two sets of minimums may also be published when
a second altimeter source is used in the procedure. When a
nonprecision procedure incorporates both a stepdown fix in the final
segment and a second altimeter source, two sets of minimums are
published to account for the stepdown fix and a note addresses
minimums for the second altimeter source.

Notes
1 Under relatively rare conditions, TERPS specifies that a ceiling is required for a specific approach, but this will be notated on the approach chart with the words CEILING REQUIRED.

As you can find the FAA chart via googling with the Key word "FAA approach minima", There is both ceiling and visibility minoma.

@user12972 There is no ceiling minimum, only the minimum altitude to which you may descend.

The other two answers are good ones & correct as far as they go, although I don't think either entirely addresses the question as posted regarding when a pilot has to have both ceiling and vis, or vis only, and the differences between ICAO and FAA criteria.
All approaches will have PUBLISHED both a ceiling and a visibility minimum.  Whether you're required to have both of those in order to begin the approach is a separate question.  (Or, in some cases, you don't have to have either one -- FAR Part 91 for instance.  Under those rules, you can start the approach having neither one.  Maybe you're just flying the approach in order to practice the missed approach, and you're okay with the fact that you almost certainly won't be able to land out of it.)
For circling approaches, every authority I'm aware of that requires having minimum weather in order to begin the approach, requires that you must have both ceiling and vis.  For straight-in approaches, some authorities require that you must have both, and other require only the published visibility.  In FAA rules, flying a straight-in "Vis Only" is generally allowed; some operators (such as the Air Force's Air Training Command, now the Air Education and Training Command, at least when I went through pilot training) require that their pilots have both in order to begin the approach.  Major airlines typically have approval to fly straight-in approaches "Vis only."
The idea is, don't start an approach unless there's a good probability that you can land out of it.  (Part 91, as described above, is less restrictive here.)
My understanding is that ICAO does the same thing, in that under ICAO rules generally, straight-in approaches may be flown "Vis Only" and the ceiling disregarded for exactly the reason that LNafzinger mentions -- the reported ceiling may not be perfectly uniform and you may well be able to see the necessary runway/lights/environment at your Decision Altitude even though you're somewhat above the published ceiling value.  Some nations may choose a more limiting rule that requires both ceiling and vis -- I think at least one nation in the Caribbean does so, although I don't remember which.
Okay, follow-up question: why publish a ceiling if operators are free to ignore it?  Answer: besides those operators who choose to require it, it's also used when considering the requirements for alternates and when an alternate qualifies.  Those rules have conditions like "if the ceiling is lower than XXX above the lowest authorized approach's minimum ceiling and/or the visibility is lower than YYY above the lowest authorized approach's minimum visibility, then..."  Rules like that can only work when the ILS and PAR approaches have the published ceiling values as Jonathan Walters posted.

So while I agree with much of your answer, approach plates for the US (falling under FAA rules, whether government or Jeppesen) do not publish a ceiling minimum.  Take a look at the charts, nowhere does it say ceiling. It simply shows the minimum altitude to which you may descend on the approach.

Also, I am not aware of an FAA requirement for a minimum ceiling in order to perform a circling approach. Do you have a reference for that?

You've answered that authority can choose wether they require ceiling or not. But as you can see the FAA standard minima table(which explain what figures mean) There is both vis and ceiling so i believe that the aiport which follows FAA regulation, they require both. And I have failed to find the ICAO standard minima table(which explain what figures mean), and the airport which follows ICAO regulation doesn't require Ceiling minima as far as i know.

One last thing, your last paragraph quotes the regulations incorrectly.  They do not refer to the "lowest authorized approach's minimum ceiling and/or the visibility..."  Instead, to calculate the required ceiling they say things like "Add XXX ft to MDA(H) or DA(H), as applicable" (quoted directly from Ops Spec C055).  This is because the chart does not specify a ceiling.  :)


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Can a passenger plane stand still in the air, or hover at a fixed location above a ground?
This Gizmodo link shows that a commercial passenger aircraft can stay at single location without any movement with respect to ground. (Is it real, an illusion, or faked?)
I have heard a friend saying that this happens for flights into Heathrow airport when the runway is busy with many aircraft required to land. I have argued with him about the holding circuit which basically requires airliner to follow a path around a airport, he is for sure that he has experienced such a thing happening when he was on board and also saw it live many times.

It requires a strong headwind. And separation is still in effect. This means that airplanes are still 5 nm out from each other.

I've seen that video before, and at no point does it stop in mid air. Aircraft as a matter of operation do not "hover", and definitely do not happen as part of a holding operation around an airport.

As is so often the case, the perspective fools those who don't understand how aircraft fly.  There is no way that the headwind was strong enough to reduce the ground speed to zero (they would not be flying the display if it was, or indeed, flying at all).  It's just an illusion.

Whose livery is that on the aircraft in the video?

Never mind, found it. Cathay Pacific

"Is it real, an illusion, or faked?" – The very article you linked to already answers this question (emphasis and annotations mine): "This produces extremely low ground speed [Note: "extremely low", not "zero"], which results from the A330's air speed minus the speed of the head wind. The combination of this ground speed and the A330's size give the viewer the sensation of a plane almost frozen in midair." So, a) it doesn't stop, it just goes slower than normal, and b) it's just the sensation of the viewer.

@JörgWMittag I have to disagree.  We have no way of knowing, but I seriously doubt that the ground speed was less than say 100kts at any point.  Look at the umbrellas and other wind speed clues.  I doubt that the wind at that height was any more than 20/30/40 kts or so.  It did not produce an extremely low ground speed.  It is 100% illusion caused by perspective.  Our brains are lousy at judging speed at height from oblique angles. If you had a fixed reference, for example under the aircraft, it would not have appeared slow at all.

@Simon The wind speed is probably considerably less than 40kts, given that people are using umbrellas at all.

Related:Holding circuit of Heathrow Airport

If you consider the AN2 biplane to be a commercial passenger plane then it is possible.  link

NO, at least not during landing. And especially not an Airbus.
Two reasons:

While up in the jet stream a 150 knot wind is normal, wind over 100 knots (185 km/h, 115 mph) near the ground only occurs in hurricanes (and tornadoes) and would be so turbulent that landing it it would be most likely out of question.
When landing in strong head wind the pilots will use higher air speed. This is because strong winds have a nasty habit of changing in strength or direction quickly and if the head wind reduced suddenly, the plane would loose lift and crash. So the pilots maintain some minimum ground speed to have a safety margin for that case.

Airbus even has a minimum ground speed warning exactly to keep the pilots out of this risky flight regime and that would be blaring in cockpit if the A330 in the video really almost stopped in midair. There must be something wrong with it (the linked article does not link to a video, only a screencap and one can't tell from screencap what is going on).

can a commercial passenger aircraft stay at single location without any movement with respect to ground?

In theory it could, with strong enough headwinds, low weight (no passengers nor cargo or baggages), deployed high-lift devices.
In the video it does not happen and I seriously doubt it has ever happened in real life.

i have heard a friend saying that this happens for flights in Heathrow airport when the runway is busy with many aircraft's required to land.

This is simply false

i have argued with him about the holding circuit which basically requires airliner to follow a path around a airport

That is correct

he is for sure that he has experienced such a thing hapening when he was on board and also saw it live many times.

Either he as flown in an An-2 (see linked question) or he simply dreamt about it.

It would be interesting to add whether a passenger may feel that the aeroplane is not moving, while actually descending.

An airplane cannot stand still in the air as it requires air flow over the wings to create lift. An airplane can be stationary over the ground if the air is moving over the ground quickly enough. A lightly weighted aircraft can fly slower than a heavy one, but even flying just above stall speed you would need a headwind well over 100mph in order for a jet to be stationary with respect to the ground.
In this video the airplane appears to be flying must slower than it actually is due to the angles viewed and other factors. It was never stationary with respect to the ground. Commercial jets don't "hover".
What does happen when there's traffic is that they are put into a holding pattern. This is generally away from the airport, not around the airport.


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What FAA license does the EASA LAPL convert to?
I have read about getting a FAA licence to 'piggyback' with a EASA PPL(A).
Is there a similar route for a EASA LAPL(A) licence.
I assume if there is then there would be the same restrictions ie Max 4 on board, No night flying or IFR flying allowed, simply VFR, which would not be a problem for me.
Basically, I am looking to take a flying holiday (vacation - let's not upset the U.S. readers) - not hour building to gain any additional licences or ratings.

Unfortunately, an LAPL license isn't ICAO compliant. This is from the UK CAA:

Licences which do comply are known as ICAO licences and those which
  do not are known as non-ICAO licences. Non-ICAO licences are not fully
  recognised internationally, and therefore are only valid for use
  within certain states or Europe; for example, the LAPL is only valid
  in Europe.

That means that the FAA won't recognize it and you can't use it to get an FAA PPL under 14 CFR 61.75, because that requires you to hold an ICAO compliant private pilot's license. As far as I know, there's no other way to convert a foreign license to a US one (a sport or recreational pilot license would be the closest US equivalent).

Added note that the FAA sport license has a limit of one passenger.


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Where does the fuel that the mid-air refueling probe collects go to?
Where does the fuel that a mid-air refueling probe collects go to? Does it go straight to the engine, to the fuel injector(s), or does it go to some place that then sends the fuel somewhere?

Since it's a refueling probe, wouldn't you expect it to feed into one or more of the fuel tanks on the receiving plane?

Are you asking about the part that the fuel plane sticks out or the part that the plane that is refueling has?

@DanPichelman On a really basic level, yes.  But the probe itself doesn't go strait into the tank, generally.  It goes into an intake device (as show in aeroalias diagram), which will deliver it to a fuel tank.

If it went straight to the engines I'd be extremely worried about the speed at which fuel is being used! Flagging as low quality. I asked my very much non-aviation/engineering mother this question and got the correct answer.

The fuel coming in through aerial refueling probe goes into the aircraft fuel tanks just like the ground fueling. The schematic below shows the fuel system of the F-16.

F- 16 fuel system; image from f-16.net
As can be seen, the aerial refueling system's fuel load goes into the same one as the ground refueling one, through the same lines. Another thing is that in case external fuel tanks are connected, they can be topped up first (they are depressurized once aerial refueling commences and fuel transfer from them stops) before the internal tanks are filled up.
The fuel goes to the center-line(fuselage) or the wing tanks, from where it goes into the reservoir tanks, which supplies to the engine, just like 'regular' fuel.

To the fuel tanks, presumably.


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Where can I find the ground service manuals for the Bombardier BD-700 Global 5000?
I am looking looking for a comprehensive ground service instructions memo/manual for Bombardier BD-700 Global 5000 which deals with the potable water and waste systems and GPU connection. Where can I find such manuals?

Welcome to Aviation.SE! This something you should probably get from the manufacturer or a professional mechanic. I'd be surprised to find it on the internet.

I second that.  If it's for personal reference, you may be able to turn up such a document using Google.  If it is for professional use you MUST consult with an OEM approved document for this purpose.

Most of these manuals are copyrighted and not available (legally) publicly. You will have to contact Bombardier or an authorized service center for the manuals (and probably pay for them).

Bombardier offers these manuals through their Technical Publications department.
For your purposes, the following product may suit your needs:

Ground Handling, Servicing and Emergency Information Booklets
  These booklets are designed to provide information to personnel that
  are not necessarily familiar with BBAS aircraft, but are called upon
  to provide servicing or emergency service (e.g. fire fighters).
  Included in the booklet is a general description of the aircraft,
  towing, jacking and servicing information, danger areas, explosive
  hazards, egress information, fire fighting information and emergency
  lifting instructions.

Alternately, you may wish to have more detailed servicing instructions, which should be found in the Service Manual:

Maintenance Manuals and Service Manuals
  The manuals contain the information necessary for a trained mechanic to perform servicing,
  fault isolation (trouble shooting) and functional testing of various
  systems. It also includes information necessary for the mechanic to
  perform maintenance on the aircraft at the Line Replaceable Unit (LRU)
  level which includes removal and installation, adjustment, cleaning,
  painting and minor repairs.

Thanks, we got all these, but the information on Potable Water, Waste and GPU connections is very scant (to say the least)...


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Why isn't tungsten used in supersonic aircraft?
Why isn't Tungsten used in supersonic jets or flights even though it has a High Melting and Boiling point?

It is a very dense metal which would make the aircraft heavier at the same size. There might be some tungsten alloys used in aircraft though.

Tungsten is used on aircraft, for counterweights, in replacement of uranium that was a bit more touchy to recycle. See a list here (while trying to close popups...) Tungsten and uranium are more dense than lead, which is a problem for aerospace for other uses.

We have a 40lb chunk of tungsten that lives in a little tight-fitting Pelican case. It's fun to hand it to someone and see people do a double take when they misjudge how heavy it is- tungsten is almost twice the density of lead.

@SpehroPefhany: I've the same problem when carrying my gold ingots to Grand Cayman, it's not like in movies.

While tungsten does have a high melting point (I'm not sure what's the relevance of boiling point here), it has little else to offer for aircraft construction. The main issue of course is weight. One of the main goals of aircraft design is to reduce weight while achieving all the required performance parameters and tungsten is among the heaviest elements around, even beating Uranium.
When you factor in its density, it appears less than ordinary- its specific strength is less than 100 KNm/kg, less than half of Aluminum. Another thing is that tungsten is not exactly an easy material when it comes manufacturability. Even its saving grace, the high melting point (over 3000$^{\circ}$ C) is not very relevant as far as supersonic aircraft are concerned. The temperatures experienced in aircraft anywhere other than engines is well within that limit. For example, the surface temperature of Concorde wouldn't even get to a tenth of it.

Concorde surface temperature; image from erau.edu
Surprisingly, tungsten is indeed used in aerospace industry- at exactly the other end of the speed spectrum. it is used as a counterweight for controls in small aircraft and also as balancing masses in rotorcraft blades.

What small aircraft uses tungsten as a control counterweight?

@JonathanWalters Here's a list from a supplier.

So, the Adams series apparently. I had heard of it used by home builders in Lancair aircraft, but that was all.

Does tungsten form alloys with lighter metals that might take advantage of the properties of both?

@TomMcW Almost all the tungsten alloys I'm aware of have high densities- used anywhere from armaments to counterweights.

@TomMcW Yes, small amounts of tungsten can be alloyed with steels to strengthen the steel.

@TomMcW Tungsten Carbide Cobalt coatings are used in a number of applications, as a thin flame sprayed coating, to increase hardness in selected areas of aircraft components.  (I have seen this application on a bearing journal for the main shaft in a helicopter transmission).  The virtue of that coating is its hardness, which leads to reduced or no wear on that area in the base metal.  A similar virtue is found in tungsten carbide coatings for cutting tools used by machinists to shape steel parts.

Why cant we use tungsten in smaller amounts preferably in the hottest points (ex: nose and leading edge) as a protective armor?

" the surface temperature of Concorde wouldn't even get to a tenth of [that temperature]" It doesn't make sense to talk about a tenth of a temperature, unless you're using kelvin (and, even then, I'm not convinced it's meaningful). The temperature of Concorde's nose (127C, according to the diagram) is 3.7% of the melting point of Tungsten in celsius, but 4.2% in fahrenheit and 10.8% in kelvin.

@Nived Why do you need armour that can cope with 3000C when your plane only heats up to 127C? Your frying pan gets hotter than that but you don't go hunting for tungsten cookware.

can it be used for hyper sonic planes?

@Nived The horse is dead. Stop flogging the horse.

@Nived, yes, it can & is; see my answer.

Tungsten is expensive, lead is not. Why is tungsten used in preference to lead for counterweights? Surely there can't be that much volume involved for the counterweight, even modest volume would be super heavy.

@Bohemian Tungsten is heavier i.e. denser than lead (19.3 g/cc versus 11.34 g/cc around). This makes it ideal for cases where smaller, specific geometries are required. Also tungsten doesn't creep like lead.

So, we should use uranium for airplanes? I don't see any problem with that. They'd have to be a kind of yellow, cake-ish color though.

@Hack-R Uranium metal is dark grey. Yellow cake is a mixture of uranium compounds (mosly triuranium octoxide) that's an intermediate stage in uranium ore processing.

While having one of the highest melting points, Tungsten is very heavy (very dense). Excess weight is detrimental in aircraft design.
Some of the parts that would need to be the most heat resistant, such as turbine blades are subjected to rotational forces that increase directly with the mass of the part. Thus a high mass material such as tungsten would increase the forces the part would have to withstand. These forces are already measured in tons per blade. Tungsten is also brittle. If a turbine blade failed, as does happen, it would be very difficult to contain. Tungsten is used in armor piercing projectiles; the results of a tungsten turbine blade failure would not be good.
Most other heat critical uses do not require high melting points, making other materials quite suitable.
Tungsten could have applications in ballast or balance weight applications, but lead usually works quite well here. On heat sensitive supersonic aircraft, tungsten could possibly be a suitable choice for this application, as lead does have a very low melting point of 327.5°C. On some of the fastest supersonic aircraft, skin temperatures could reach 200-500+°C, making lead unsuitable for these applications.

Fun fact: the name tungsten comes from Swedish tung sten, meaning heavy stone.

In extreme flight conditions (i.e., hypersonic flight), it is; the nosecap of the X-51A was tungsten (PDF). Its density just isn't worth it at mere supersonic speeds.

Being at the extreme end of a scale (see: melting point) isn't necessary to cross the threshold of being the most useful. For example, the B-2s carbon-graphite frame is stronger than steel, less dense than aliminum, and has a melting point of 3,500 degrees C, whereas tungsten would melt at about 3,400 degrees C. Both of these are far higher than necessary even if you account for gas-turbine compressor combustion, but reducing weight reduces work and increases speed.

Worth noting that, at terran atmospheric pressures, carbon doesn't melt at all - that 3500C figure is its sublimation point.


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Do any commercial airline models have a parachute at rear to help in landings?
Some land-based fighter aircraft have a drogue-chute to help them slow down during landing.

Do any current models of commercial aircraft have a similar
mechanism?
Secondly, would such a mechanism help aircraft in making a better landing and reduce landing related incidents/ accidents?

One thing to note: You'd have to be very committed to the landing before deploying the chute. Go-arounds would be very problematic while pulling a chute behind you. By the time you've fully committed to stopping, it seems like the chute would be of limited usefulness, except perhaps in emergencies like Lnafziger mentioned.

Almost all landing accidents where the aircraft has reached the runway are as a result of the PIC making a decision to land when they should not have.  No retarding system in the world will help with that.

@reirab: Simple solution - when going around, cut the chute.

It is called a drogue parachute and I don't think that any of the present generation large commercial aircraft use them.
That said, a number of older commercial aircraft have used them, a good example being the Sud Caravelle.

Image from eu.airliners.net
There are some issues with using drogue chutes in a commercial airliner, which would limit their effectiveness.

Most commercial airliners have thrust reversers already.
For it to be effective, the parachute has to be quite huge. For example, the Handley Page Victor, which had MTOW in the range of A320, had a drogue chute 48' (or 14.6m) in diameter. The chute would occupy volume and increase weight.
If drogue chutes are used, they have to be packed before every flight, which will increase turnaround times. Another option is to replace them at the end of every deployment, which would increase costs.
Deployment failure/partial deployment/Uncommanded deployment will affect the safety of the flight.
The need to use a drogue chute can limit the crosswind landing of the airliner significantly.
Aircraft dragging a huge chute during taxiing back to the ramp is not a good idea from FoD point of view.

That's quite an amazing picture! I can imagine that using that chute would only be practical if there was some sort of pre-packed canister that would allow it to be quickly replaced during routine turnaround procedures, while the used chute & canister were sent off to a central facility for inspection & repacking.

Was it used regularly for every landing, or only on shorter runways?

Other old, smaller "commercial" jets (like the Learjet 20 and 30 series aircraft) also have a drogue chute.  It is intended only to be used in an emergency when a landing goes awry (brake failure, off-field landings, poor braking conditions where it appears that you could run off the runway, imminent collision, etc.)

"If drogue chutes are used, they have to be packed before every flight... Another option is to replace them at the end of every deployment" An intermediate option, in terms of both time and cost, would be to take off the used chute, install a freshly packed one and then repack the used one for re-use on a later flight.

Non copyrighted image https://en.wikipedia.org/wiki/File:Air_France_Caravelle_with_parachute.jpg

@Lnafziger: That seems like it would be most useful, not in a landing, but in a high-speed rejected takeoff.

@Sean That as well!

Also this photo

The full list of airliners and business jets is:

Sud Aviation Caravelle
Tupolev Tu-104
Tupolev Tu-124
Tupolev Tu-144
Concorde prototypes
Learjet 25
Learjet 35
Dassault Falcon 20

Concorde during brake tests (picture source)
Even some gliders used them (SHK, HKS, SB-5, Salto)

Hirth SHK with brake chute (picture source)
Brake chutes increase drag and allow a steeper approach, which helps to control the touchdown point with great precision. If the aircraft is too high, just push the stick forward - the drag of the chute prevents the aircraft from accelerating too much and the increased drag at higher speed gets rid of the excess height quickly.
On the ground the drag of the chute reduces the rollout distance which is especially helpful for aircraft with a high touchdown speed.

With the improvement of the brakes and reverse thrust, parachutes are no longer an option for airlines. Nowadays the carbon brakes are the best brakes in the aviation market, together with auto brakes, reversers and speed brakes, most of the new generation aircrafts are landing much shorter compared with old generations, the reject takeoff is almost automatic and with the correct calculation the runway overrun accidents are decreasing considerably. As the effectiveness of the thrust reversers are better at high speed, most of the airlines uses reverser thrust in idle, the pilots are suggested in case of using other than idle reversers, to apply reverser thrust until 80 knots or 60 kts after landing, auto brakes combined with Carbon Brakes and a speed brake, give to the aircraft shorter stop distance than before. The brake temperature is much better in carbon brakes, hot temperatures are still a problem in case of reject takeoff but for long taxis is no longer a problem. So parachutes are retired in commercial aviation. Hope this explanation helps. ;)


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Where can I find a list of past flights at LHR?
I am doing a project based on some data recorded from London Heathrow during October 2012, to quality control the data I need to know when aircraft flew past the detectors.
Are there any online databases of when flights took off and landed at Heathrow during this period?
If the data contains which runway the aircraft landed on that would also be desirable,
Thanks for the help

If you're lucky, you might just get it for free (see halfway down this page):

We have provided a data drop of historical flight information for
  specific airports or airlines, on a one-off basis.

As for the runway data, it's harder, since I don't know of any website which records it. You can have a look on Flightradar24 for which runway is in use at the moment depending on the aircraft positions, but apart from that it's probably not going to be easy. You might want to email the airport directly and ask if they could provide a figure for that month for each runway, but I'll doubt that a single source has the details for each and every flight.
If you get to it, have fun sorting out 40,000 flights ;)

Manfreds answer will hopefully help you  finding the data you need. On this website you can check which runway was in use that day, but!! only if you know if they were on east or west operations. There are probably enough stored weather data to find out ;) To me it sounds some kind of strange what you are doing - okay, I decided to fly planes rather than to study - but maybe you tell us the reson. Here you go:  http://www.plane-mad.com/airport-spotting-guides/united-kingdom/london-heathrow-lhr-egll.html?force-nonmobile=true#runway-alternation


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What is the use of this sharp edge found on the leading edge of a DA 40NG wing?

It is found on both wings and there is also another long one near the wing root too.

I'm leaving the aerodynamics to someone who paid more attention in classes but, as I remember, that leading edge is supposed to induce flow separation at lower angles of attack (for that portion of the wing), inducing a small buffet into the ailerons and a buffet feed-back to the pilot through the control stick.
Without it, the wing would begin separation (and stalling) at the root, and by the time you get the buffet in the stick the whole wing is basically already stalled.

Stall strip. But shouldn't it be located near the root rather than near the tip.

there is a larger one near the root (as you want the root to stall first), also a small one along the ailerons to produce buffet

It's actually the correct answer! If the Angle of Attack rises to much then this little edge will produce turbulent air which hits the elevator and produces notable feedback in the pilots control stick.


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Is it possible to fly a Straight and Level flight at high speed?
The question arise while discussing with some of my student pilot friends that as we have been performing Slow Flight we have to keep increasing the AOA of the aircraft to compensate for every lose in airspeed. What about "FAST Flight", decreasing the AOA of an aircraft for every gain in airspeed. Maybe Negative AOA.
I'm aware that lift could be generated with negative AOA from the graph of AOA vs CL. And if you could explain to me why there is a specified cruise setting?

Straight and level is usually a term meaning "holding altitude and heading", not necessarily the fuselage being level with respect to AoA. So the answer to the title of the question is yes, unless you run out of elevator authority or lift you can fly "straight and level" at any speed

Yes, with good engines and low drag you can fly straight and level at really high speeds.
What about the AOA? It gets smaller, but only down to a certain limit. At level flight, the weight of the aircraft is equal to the aerodynamic lift:
$$ W = L =C_L\cdot\frac{\rho}{2}V^2\cdot S $$
The lift coefficient depends on the AOA ($\alpha$):
$$C_L=C_{L0}+C_{L\alpha}\cdot \alpha$$
Therefore,
$$\alpha = \frac{1}{C_{L\alpha}}\left(\frac{2W}{\rho V^2 S}-C_{L0}\right)$$
For very high speeds the first term of the parenthesis will get close to zero, so that the desired AOA setting becomes
$$\alpha_{V\rightarrow\infty} = -\frac{C_{L0}}{C_{L\alpha}}$$
This AOA is indeed negative for most aircraft. If your AOA gets smaller you won't be able to sustain level flight.
The plot below shows the AOA needed for straight and level flight on a Cessna 172 and compares the two terms in the parenthesis of the equation for $\alpha$ (multiplied by 10 for better readability). You would need to fly much faster than $V_{NE}$ to get the first term significantly smaller than the second one.

Source: own work with data from Wikipedia and polar.

In order to fly at top speed with the fuselage level you need to pick the right altitude.
Consider the picture below: It shows a Northrop B-2 (foreground), a Rockwell B-1 (midlevel) and a Boeing B-52 (background) flying in formation at moderate altitude. The fuselage attitude is considerably different between all of them.

Northrop B-2 (foreground), Rockwell B-1B (midlevel) and Boeing B-52 (background) flying in formation (picture source)
Why the difference in fuselage attitude?

The B-2 is designed for midlevel cruise and has a fairly high aspect ratio, which translates into a decent lift curve slope. Here it flies close to its design point and looks about level. A high lift curve slope means that small angle changes suffice to correct lift for off-design situations.
The B-1A was designed for supersonic flight first and was modified for subsonic low-level penetration in the B-1B. Here it flies in less dense air and slower than its design point, and you can see that it needs to keep the nose up. This is amplified by the high sweep angle and low aspect ratio with backwards-folded wings, which results in a low lift curve slope, so a relatively large angle change is needed to fly slower and higher than designed.
The B-52 in turn was designed for very high subsonic flight, so here it flies in much denser air which requires it to fly with a negative fuselage attitude to limit lift.

Each of the three will fly with the fuselage about level in its design point. This can be generalized for any airplane: All what is needed to fly fast with a level attitude is to adjust the flight level to something close to the design point.


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What are the advantages of single vertical stabilizer over a twin set?
Why did the PB4Y patrol aircraft use a single fin while the B-24 it was derived from used twin fins?  I have read that the twin fins were thought to be superior but this was realized to be false.  Why did engineers believe the twin fins to be superior and what caused them to reconsider?

Why did engineers believe the twin fins to be superior?
There doesn't seem to be any single definitive reason for selecting the number of vertical fins on an aircraft design (except, obviously, for the twin-boom designs such as the Lockheed P-38 or Fairchild C-119).  I've seen all of the following cited by one authority or another as advantages of multiple vertical fins.  (Sorry, I didn't save or remember the sources.)

Reduced hangar overhead requirement.  This is the most common reason for including total aircraft height in a purchase specification, and it is a sine qua non requirement for carrier deployment.
Also I remember reading a magazine article that supplied this reason for the triple tail on the Lockheed Constellation.

Reduced cantilever stress at the root.  This may actually save weight over the single-fin design.  It may also help keep a tactical aircraft from disintegrating under the enormous stress of combat manoeuvres.

Reduced parasite drag.  The lower half of a vertical fin can be twice as thick as the upper half.  A twin tail design effectively omits the lower half and uses the upper half twice.

For multi-engine designs, operating the rudders in the engine slipstream affords improved rudder authority at low speed.  Pilots who taxi loaded bombers into takeoff position between other loaded bombers like lots and lots of rudder authority.
Also I notice that practically all the big flying boats of the 1940's Flying Clipper era had multiple vertical fins.  Those craft had some tricky low-speed steering requirements — they not only had to make their way through a busy harbor, but usually docked and undocked without a tugboat.  I'm only guessing at the connection though.

A single vertical fin can be masked by the fuselage wake at high angles of attack.  You can see how this could be a problem for modern fighter aircraft, which look like flying tennis courts.  Traditional bomber designs with their broad wings and heavy aerodynamic loading can also generate quite a troublesome fuselage wake.

Improved position and field of fire for a tail gunner.  I don't see it myself but I've heard the claim so I'm including it here.
I concede that the tail gunner slot in the Avro appears to benefit from the absence of bracing structure.

Reduced rudder-induced roll due to a shorter rudder offset moment.  Fast tactical military aircraft have short wings, and you don't want the rudder to extend further from the roll axis than the ailerons do.  On the other hand, the rudder doesn't need to go all the way to the top of the fin, does it.

Twin fins can be canted to reduce radar return.  They usually are, although I don't understand how the designers know where the hostile radar will be.

On the A10, the vertical fins actually mask the hot engine exhaust over a large angular area.  Thus in order to aim a ground-to-air heat-seeker up the tailpipes of an A10 you have to be directly behind it.  Which means that a few seconds ago you were directly in front of it, so good luck with that.

Is hangar height such an issue for carriers? They already have folding wings, rotating the tail sound easier. No hinges in the middle, just rotate the whole assembly to one side.

+1 for a solid analysis, but the last sentence takes the cake.

One more... The AN-225 had a twin-fin design (compared to the AN-124 on which it was largely based) becaused the Soviet shuttle on top would've masked the original fin (it would not be effective in the turbulent air caused by the load). On the 747 which ferried the US Space Shuttle, the designers added secondary fins on the ends of thehorizontal stabiliser.

@T.J.L.: Although it does assume that the A-10 had seen you while you were in front of it, which might not be the case if the MANPADder is well-hidden.

The Consolidated B-24 Liberator was designed in something of a hurry. In fact, the Consolidated company had told the USAAF (who had asked them to produce B-17 under license) that they would have the aircraft ready in the same time it takes to adapt B-17 for production in the Consolidated plant.
As a result, the parts of design were 'borrowed' from other aircraft. The twin tails were taken from the Consolidated XP4Y Corregidor.

"XP4Y-1 in flight" by Unknown - This image is available from the United States Commons.
Apparently, the B-24 (which was Consolidated Model 32) used parts straight off from the XPFY (which was Consolidated Model 31), with many parts in vertical fin starting with the 31 prefix of the XPFY. Reportedly, some parts used were from other Consolidated Models, the PB2Y Coronado Flying boat and PBY Catalina.

Image from acepilots.com
More B-24s were produced than any other heavy bomber but it had its own share of problems. After sometime in service, the USAAF determined that the aircraft would have better stability if it had a single vertical tail and a production aircraft was modified (called XB-24K) for testing this. Tests showed the following improvements:

Enhanced lateral stability and improved handling characteristics.
The design was better able to cope up with two engines out on one side condition.
Increases in field of fire for the ball turret, waist guns and top and tail turrets.

The army was impressed enough that it recommended that single tail be used in all future aircraft, with the model number B-24N. The B-24N also included other improvements like the installation of spherical Emerson model 128 turret in the nose, modified (knife edge) windscreens etc.

Image from edwards.af.mil
An order for over 5000 B-24Ns were placed, which were canceled later (in May 1945) as the war drew to an end and only a handful of B-24N with single vertical tails were made.
The PB4Y-2 Privateer was developed from the later models of B-24 and had the single vertical fin in its design as a result. Most of the PB4Y-2s were in fact, delivered after the WWII.

"PB4Y-2 Privateer VP-23 in flight" by USN - Dictionary of American Naval Aviation Squadrons Volume 2: The History of VP, VPB, VP(H) and VP(AM) Squadrons 1; U.S. Navy National Museum of Naval Aviation photo No. 2011.003.156.015. Licensed under Public Domain via Commons.

My answer may be "nice" but it doesn't really respond to Brinn's main question about the PB4Y-2.  Your answer does, and should be accepted.

The additional weight associated with control surface hardware like cables, hinges, a beefy horizontal, etc must be a consideration.  The added complexity related to maintenance of the system might be another.  On the other hand, battle damage that might completely obliterate a single vertical may only damage one of the two on a plane equipped with a pair of verticals.

There are multiple reasons for twin vs single fins. In the world war 2 era, twin fins were used to help with engine out performance. The blue line speed on WW2 twins might be 50 mph faster than the take off speed, and even on four engine designs it was a major factor because bombers on take off were heavily loaded. Lose an engine and it would be a very close thing 4 engined or 2. Twin fins helped in this respect by lowering the blue line speed over single finned designs.

This answer could be improved by a brief parenthetical comment explaining what blue line airspeed is.  Presumably minimum controllable airspeed at full power with the most critical engine failed?  Also I assume you mean " In the world war 2 era, twin fins were used to help with engine out performance by putting the fins in the propwash of the inboard engines"?  Or, perhaps not?

I have looked up the definition of blue line speed on the internet and it says "speed for best rate of climb on one engine in multi engined aircraft". There is some debate over if it should be best rate, best angle, but the point I'm making in my answer is that as airspeed slows on one engine, full rudder deflection will be needed and as it continues to slow a loss of control will occur, which is why pilots are told to aim for the blue line speed because there lies safety if an engine were to fail. So the slower the speed that occurs, the better the aircraft design.

Well, the double vertical stabilizer is still in use in many airplanes like the F-22, F-35, F-15, A-10, and the A300 Super Transporters as well as many other airplanes. So, there is not much "reconsideration" in aircraft design on twin or single stabilizers. The reasons for choosing one over the other is survivability (some military aircraft) or structural requirements compared to simplicity.

This does not appear to answer the actual question, twin vertical stabilizers are rare nowadays and seem to be pre-dominentaly on planes that don't support a single vertical stabilizer due to the airframe covering it (the mentioned Airbus Beluga for instance)

@JustSid, also on the Shuttle Carrier due to drag and turbulence introduced  by the Orbiter.

@JustSid The last sentence does sort of answer the question, though it's not a particularly detailed answer.

Twin fins/rudders reduce yawing from landing or taking off in a crosswind as the second one is blanked and the affected side area is smaller. Twin tails are in the prop wash of the engines on multi-engine aircraft and so are slightly more effective especially at low speeds, and can thus be smaller than fins/rudders that are not. Some aircraft used multiple tails to reduce their height so as to fit into smaller hangars such as the Lockheed Constellation. Twin tails used as endplates on the tailplane also increase the tailplane's efficiency so it can be smaller and produce less drag. As mentioned however - they are more complex, and in an engine out situation yaw control may be degraded slightly. The only aircraft where redundancy was claimed was with the A-10 and that claim is dubious as the rear fuselage remains a weak point. Similar aircraft would be highly dependent on how the controls are designed. The B-24 had snaking issues that could have been resolved by lengthening the fuselage slightly but they chose to go for a redesign.


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What is the relation between aircraft descent rate and speed?
I am an ATCO working in India. I am trying to understand why an aircraft's rate of descent increases, when its speed increases. I am particularly interested in how it relates to jet engine aircraft.
Is it the same for aircraft climb rate?

Is this an observation you make when you instruct a higher speed to a descending aircraft?

If the pilot commands a certain glide path angle, any change in airspeed will also change vertical speed, because both are connected by the glide path angle $\gamma$.

Note that the vertical speed is air speed$\cdot\sin\gamma$. Climbing or descending makes no difference; only the sign of $\gamma$ changes (positive $\gamma$ means the aircraft climbs). Also the type of propulsion will not affect this basic relationship.
Since descending reduces potential energy, some other energy will increase. If the aircraft speeds up, its kinetic energy increases until the increase in drag balances the energy gain. Reducing engine thrust or deploying spoilers will limit the speed increase, but for a steep descent, even with idling engines and full spoiler deflection the flight speed will increase.

Is it the same for aircraft climb rate?

No. In a climb the potential energy must be increased, and the primary source of energy are the engines. Only now will the type of propulsion affect the result: Both the speeds for the steepest and the fastest climb for propeller aircraft are fairly low, so an increase in vertical speed will normally be accompanied by a reduction in flight speed, freeing up more excess energy for climbing.
Jets also need to fly slow for their steepest climb, but need to pick up speed for their fastest climb speed. This is the more pronounced the lower the bypass ratio of the engines is, so fighter jets need speed to climb quickly. Turbofan-powered airliners fall between propeller aircraft and fighter jets and prefer a moderate speed for their best climb performance.
This is all only true in stationary flight. In a pull-up, any aircraft can gain altitude quickly by trading kinetic for potential energy. But this is a transitionary effect - the high climb speed will only last while the aircraft has excess speed to spend.

To add an ELI5 to this: First walk down a hill. Then run down the same hill - which technique gets you to the bottom of the hill fastest?

Here's Peter's diagram with two different airspeeds, so you can see the difference in the vertical speed:

The other answers handle the question very basically. If a plane is moving through the air in the direction of its nose, and that nose happens to be pointed toward the ground, then the faster the plane is moving through the air in the direction of its nose, the faster it will descend. This is simple trigonometry; the plane's "true airspeed" is the hypotenuse of a triangle, the other two legs being the pure horizontal and vertical components of that airspeed, namely the plane's "ground speed" and its "vertical speed" (aka climb/descent rate). At a constant angle of climb or descent, if you increase your airspeed, you will increase your ground speed and your vertical speed.
Another, more realistic way to think of it is the other way around; all other things being equal, when an aircraft's descent rate increases, its speed increases. This is due to physics, and the conversion of energy between potential and kinetic forms.
A simple non-aircraft example. Say you have a sandbag with a rope tied around it and fed through a pulley overhead. By pulling on the rope, you lift the sandbag against the force of gravity by a certain amount. You expend energy to do this, and are storing some of that energy (ideally, all of it, but in the real world there are "inefficiencies") in the weight inherent in its height above the ground. Release the weight, and gravity accelerates it by a time-dependent (and thus height-dependent) amount, so that by the time it hits the ground, all the potential energy you stored by hoisting the weight has been converted to kinetic energy of motion.
This same basic relationship also exists in aircraft; an aircraft stores energy expended by its engine (or in a glider, by the engine of a tow craft) in its altitude. The aircraft can then reduce its altitude to maintain (or in a true dive, even gain) forward airspeed. This is one of the most basic principles of aerobatics and is taught to all pilots; you can "trade altitude for airspeed", whenever you need more (or less) of one and have enough of the other to give up (or gain).
The mechanisms by which this happens can be made as complex and esoteric as you want to get; we can cover the four fundamental forces of flight and how different forces act in different directions depending on the attitude of the aircraft and thus contribute different horizontal and vertical subcomponents to the aircraft's motion relative to its nose or the ground under it. But, to answer this question it is enough simply to state that as a plane descends, it is trading its altitude to gain energy, and the amount of that energy not lost to drag will increase the plane's forward airspeed.
This feeds back into the original answer. As you descend from altitude, your forward airspeed (the speed of travel in the direction of the plane's nose) will increase as potential energy is converted to kinetic. Because that forward airspeed is in a downward direction, as long as drag is not keeping acceleration in check (such as with a higher angle of attack, which increases drag by increasing the "cross section" of the wings directly exposed to airflow), it creates a feedback loop; you descend, gaining airspeed, which causes you to descend faster, gaining more airspeed.
To counteract this, descending airliners often reduce engine power (so the energy gained by descending merely offsets the decrease in thrust), increase their angle of attack (if the aircraft is travelling along a path "beneath" its nose, the wings are more "side-on" to the airflow moving past them which increases drag) and deploy spoilers or air brakes (which directly increase drag even at low angles of attack).

So unlike the answers that were here first, I say that the short version is that "out in the real world", it doesn't work that way.... except in some situations that are commonly seen as an air traffic controller.
When a pilot is given a descent, with no additional instructions, we will typically descend either at a constant rate of descent or a constant airspeed.  The AIM says that we should descend at an optimum rate (not the maximum rate) until within 1000 feet of the assigned altitude, and then no more than 1500 fpm for that last 1000 ft.  Depending on my distance from the destination or other expected crossing restriction, I typically choose a descent rate between 1500 and 2500 fpm and use power to maintain my target descent airspeed.  When using this method, the descent rate will not increase with speed, however my vertical flight path angle will change as the speed changes.
The behavior that you describe is most likely seen when a crossing restriction is issued to the pilot, or they are planning to cross a self-made crossing restriction (10,000 ft. AGL at 30nm from the airport is commonly used).  This effectively fixes the vertical flight path angle, and now the only way to compensate for speed changes is to adjust the vertical speed.
When descending from high altitudes (above 30,000ish feet, depending on the airplane), the indicated airspeed will increase with a constant Mach number, until the target descent airspeed is reached, and vertical speed must be increased in order to compensate for the higher ground speed in order to make the crossing restriction.  On the other hand, if a speed reduction is assigned then the descent rate may be reduced in order to make the same crossing restriction (or the aircraft just reaches it's assigned altitude early, which is not normally an issue).
This adjustment happens either automatically (by the FMS / autopilot in VNAV mode), or is monitored and manually adjusted by the pilot in order to meet the crossing restrictions.


text:
How does an engine fall off the wing?
When I was a kid, I remember one day the news on TV said that a passenger airplane lost an engine.  It simply fell off the wing to the ground.  Then they showed the plane (which landed safely) and the bent up engine in some field.
1) How could that happen that an engine simply falls off?  How many bolts are holding the engine up?
2) If it sheared off the wing, why didn't it break the wing with it?
3) Are there any special protocol(s) for missing engine v. disabled engine?
In addition to what I saw as a kid, this happened last year

Related: Have jet engines ever “Sheared” off?

Whoever is voting to close this, I wouldn't say it is a duplicate because in that question I am asking for incidents where engine-off happened and not how engine-off "works". None of the answers there would answer this question.

The pylons are actually made so that if they have a problem, such as vibration that could affect the rest of the plane they will fall off do they don't take the wing with them. Loss of an engine can be dealt with but a damaged wing is a much more serous problem

Related to the 3rd question, specifically regarding the fuel lines.


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Why is there no "reverse sensing" on a horizontal situation indicator (HSI)?
When we use this awesome instrument for navigation (not only for ILS/LOC), it is said that there is no reverse sensing (except for back course thing). I kind of understand why pilots are confused when we use conventional VOR, and now I want to know why there is no reverse sensing? How does it work? Can anyone help me out?

For conventional VORs, Positive sensing occurs when the headng currently being flown is on the top half of the VOR.  Reverse sensing occurs when the current heading is on the bottom half of the VOR.
Taking this theory to the HSI where the currently flown heading is always on the top half and you can never have reverse sensing.
It is also proper technique to set in the front course when flying a LOC/BC approach.  This will negate the effects of reverse sensing for HSIs.  To get all my students into the habit of setting in the front course for HSIs I teach to always set in the front course even when flying on conventional instruments.

I don't believe in positive and reverse sensing anymore.  A discussion for another day.
VOR Navigation

Essentially because you turn the instrument upside down and read it normally.
On the back-course, the signals are reversed. That is a property of the ILS signal. When using traditional ILS/LOC instrument, the needle moves right when it senses more of the right signal, but since the signal is now on the left, you get reverse sensing.
On the HSI, the needle still moves right when it senses more of the right relative to the arrow when it senses more of the right signal. However, the needle points in the selected heading and is rotated with compass. So when you fly a back-course, you dial the normal course of the ILS and since you actually fly opposite direction, the arrow points down and right relative to it is left. And you get normal sensing.
For VOR, the situation is similar. In VOR, the needle moves normally if you are flying the heading selected and it moves in reverse if you are flying the opposite heading. And when you are flying the opposite heading, the HSI arrow points down, reverting the sensing back to normal.

This is true for VOR sensing as well, which is what I understand the question to be primarily in reference to. The question indicates that the OP understands that HSI reverse sensing is possible for ILS back course, but is inquiring why it is not possible for VOR sensing, as opposed to a conventional CDI navhead. This answer, while already good, could be made even better by explaining why this is true for VOR sensing.

I guess what the OP means is that why there is no "reverse sensing" switch on the instrument. The answer being rather simple - to avoid the pilots making a mistake when they forget to flip the switch to the correct position.
You can always flip the OBS 180 degrees to make that change. I like to think of the HSI as always sensing "positively". You're just tracking the wrong side.
The instrument always tells you the relative position to the VOR course / localizer, it is up to the pilot to manipulate the plane to the desired position. It's reverse tracking, not reverse sensing. The instrument tells you your deviation from the selected course, it has been and will be doing that all the time. If your selected course is 180 degrees off, following the rules of geometry, you'll naturally end up switching left and right.
It is worth noting that the autopilots have an "reverse course tracking" switch:

If you want an instrument which tells you how to fly to intercept the course, it is called a Flight Director.


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What is the purpose of the hole in the Cirrus SR22's parachute?
Watching this video I notice there is a big hole in the middle of the parachute, at its top when deployed. This feature seems to exist in other smaller parachutes.
For me, it seems that air can escape the parachute by this hole and thus the parachute is less efficient than a same-sized parachute without this hole. But it must serve in a way I don't imagine. What is its purpose?

That's a spectacular picture of the 82nd Airborne jumper you found!

For me, it seems that air can escape the parachute by this hole

That's the point, air HAS to escape the parachute anyway, and without the hole it would do so laterally and in an uneven fashion, leading to lateral oscillations.
The hole allows the air to escape in a controlled location, avoiding undesired and possibly dangerous oscillations.
Some (most personal ones, but for example not the SR22 one) parachutes have handles that allow the user to deform the parachute: this allows some extra air to escape from the desired side, giving lateral controllability.

In one sense, the 'parachute with a hole' is a derivative of the wind sock. The hole and the tapering help stabilise the sock / parachute.

In order for a parachute to move through the air, all of the air underneath it needs to move out of its way.  If a concave round parachute didn't have any kind of hole in it, it would fill with a convex mass of high-pressure air which would push air that was underneath it outward.  If it had an overly large hole, the air pressure would be accelerated inward but wouldn't face much resistance going there.  If a parachute has a properly-sized hole, the pressures that would push air inward and those which would push air outward will roughly balance, with the effect that as much air as possible will remain underneath the parachute as long as possible thus blocking its downward motion.
To be sure, it's not possible for a parachute to prevent all of the air below it (or even any of it) from moving horizontally at all, but reducing the outward pressure will greatly increase the time required for the air below the parachute to get out of its way, and thus improve the effectiveness of the parachute.


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In what circumstances do passengers or flight attendants have to take their shoes off?
Three shoes-related facts:

A passenger and a flight attendant in this Аэрофлот/Aeroflot aircraft have their shoes off or are ready to take them off:

It's suggested that shoes are taken off for evacuation.

In United 93 film, an attendant can be seen seated in the jump seat without her shoes during the climb phase.

I'm wondering whether there are safety policies applied here. What are the regulations, policies or best practices about:

Crew taking off shoes in critical flight phases?
Passengers asked to take their shoes off?

In case regulations strongly vary by country, US/Russia/EU can be taken as examples.

what about there being no requirement but both just finding it more comfortable (to which I wholly agree).

Passengers are usually asked to remove their high-heeled shoes during an emergency evacuation as they may damage the slide. In other cases, it is better to leave the shoes on during emergency as once you are outside, you're worse off without them. The usual policy is to remove only the high-heeled shoes. For example, FAA says:

FAA Policy. The FAA believes that the proper procedure regarding shoes during an aircraft evacuation is to leave them on. In the case of a forewarned evacuation it is desirable to remove high-heeled shoes and stow them in an approved stowage area such as an overhead bin. They should not be placed in the seat pocket.

This was the result of cases where the passengers sustained injuries while evacuating as they wore no shoes. For example, during the crash of Southern Airways Flight 242, the NTSB noted:

The feet of a number of the survivors were cut and some were also burned because they had no shoes for protection. In accordance with standard evacuation procedures, the flight attendants had briefed the passengers to remove their shoes to prevent damage to evacuation slides.
Had pillows and blankets been distributed and had shoes been worn, some of the passengers' injuries probably would have been less severe and more passengers probably would have been able to escape from the wreckage.

As for removing the shoes during flight, some regulatory authorities are against it, like the Australian Civil Aviation Safety Authority:

The most common injuries to feet during accidents or emergencies can be prevented by wearing suitable footwear. ... Avoid sandals and high heeled dress shoes. Keep your footwear on during the flight.... High heeled dress shoes will have to be removed before using the escape slide.

As are some airlines, like the American Airlines:

American may refuse to transport you, or may remove you from your flight at any point, for one or several reasons, including but not limited to:
...

Are clothed in a manner that would cause discomfort or offense to other passengers or are barefoot.

I understand that there was an old rule to this effect, at least in US. Having the shoes on and taking them off both have their advantages and disadvantages. Except when challenged by a FA, I guess you can have/remove your shoes as you like (As long as the one sitting next isn't complaining). If the cabin crew asks though, its better to comply.
As far as the cabin crew is concerned, I'm not aware of any rule which says when they have to remove their shoes, so I guess its upto them, though it would be better for them to leave it on so as to take care of any sudden requirements. IATA recommends shoes which won't come off easily:

Cabin Crew Uniforms
... The  following  is  intended  to  serve  as  a  guideline  for airlines
  when selecting new uniforms for their cabin crew:

Shoes or boots should be flat or low-heeled and closed with laces or traps so that they will not be lost or thrown off;

Interestingly, this forum indicates that some airlines (including Aeroflot) require their FAs to wear high heels outside aircraft, which are changed to slip-ons (or other manageable ones) once in aircraft. From a news article:

... female flight attendants working for one of the world’s largest carriers, United Airlines, are required to wear shoes with a heel height of between one and three inches. Women crew members can only switch to a lower heel—it still has to be at least one-fourth of an inch high—during the service portion of the flight. ... Female attendants then have to switch back to the higher heels once the flight begins its initial descent.

Female attendants then have to switch back to the higher heels once the flight begins its initial descent. Very interesting - an accident is much more likely during descent/landing than during cruise, thus it's more likely they would have to take them off to use the emergency slides & risk foot injury once out of the plane.

Wow, nice research and interesting factual post. Thanks.

If you've ever walked up and down the aisles of a crowded jet while the lights are down, you know why it's advised. Lots of debris (starting with those little metal toy cars) on the floor, as well as clothes/blankets, means much more likelihood of slippage.

Hmm, I'd imagine you'd advise people to remove shoes in the event of a water landing or when they're likely to end up in the water, rather than people trying to untie wet shoelaces. But different instructions for different scenarios is probably deemed too complex for passengers to absorb and execute effectively in an emergency.

Another example of the problems with taking off shoes (although fortunately no injuries were caused thereby): "About a third of
the passengers who completed questionnaires stated that shoe removal either
slowed their evacuation or that shoes in the aisle obstructed their exit.
Passengers (including a woman carrying a 10-month old baby) reported that
they stumbled or tripped over piles of shoes in the aisle and galley areas." (AA1572, November 1995).